Continuous casting ingot mold for metals, and system and method for break-out detection in a continuous metal-casting machine

ABSTRACT

Disclosed is a continuous casting ingot mold for metals, of the type made of an assembly of metal plates mounted against cooling devices configured to allow cooling of the metal plates by the circulation of a liquid coolant. Said ingot mold comprises an optical fiber with a diameter greater than 1.6 mm, having a plurality of fiber Bragg grating filters and extending in a wall of at least one of said plates in a direction that is not parallel to the axis of casting of the ingot mold.

The invention relates to a continuous metal-casting plant. More particularly, the invention relates to an ingot mold for continuous metal casting. According to other aspects thereof, the invention relates to a system and method for detecting breakout in a continuous metal-casting plant.

A continuous metal-casting plant, for example a continuous steel-casting plant, generally comprises an ingot mold into which a liquid metal is poured in order to solidify it in a suitable shape. It may be a bottomless ingot mold, in which case the metal cools so as to form a slab. To cool the liquid metal, walls of the ingot mold are attached to cooling devices, for example of the liquid cooling type. The ingot mold and the cooling devices are dimensioned in accordance with the rate of flow of the metal such that the slab, when it exits the ingot mold, has a solidified external surface with a sufficiently great thickness to trap the metal that is still liquid in the core of the slab.

As the liquid metal flows in the ingot mold, it is possible for the metal to stick to the walls of the ingot mold; this is not desired and may have significant consequences for the production of the plant. This causes in particular the well-known phenomenon of breakout. The sticking of the metal to the wall creates a region in the slab in which the metal does not solidify properly, such that the slab exits the ingot mold with an insufficiently thick external surface in this region. As a result, it tears and allows the metal that is still liquid in the core of the slab to flow out. Besides the yield loss, the liquid metal, which is thus at a very high temperature, may damage the plant or even constitute a risk for operators of the plant. It is therefore necessary to detect these breakouts as soon as possible in order for it to be possible to take preventative measures, for example to slow the rate of extraction of the slab, to temporarily stop the plant or any other corrective measure.

A method for detecting whether the metal is sticking to the walls of the ingot mold, which is a sign of an imminent breakout, is known from the prior art. It is based on the measurement of the temperature of the walls of the ingot mold at different points. Specifically, it has been observed that the walls have a particular temperature profile when the metal sticks thereto. A known means for measuring this temperature consists in installing thermocouples regularly distributed on the walls of the ingot mold so as to be able to detect any temperature anomaly as soon as possible.

This detection method is advantageous but poses a number of problems. Specifically, in order to be able to measure the temperature of the walls at a maximum number of positions, it is necessary to install a large number of thermocouples. This not only increases the manufacturing cost of the ingot mold but also makes the electrical connection of the thermocouples complex. Furthermore, the thermocouples do not always make it possible to effect a precise and reliable measurement of the temperature of the walls, and so they can generate an unsatisfactory number of false alarms, that is to say alarms that signal an imminent breakout when this is not the case.

Another problem is linked to the configuration of the ingot mold, which is normally formed by an assembly of metal plates bearing on cooling devices configured to allow the cooling of the metal plates by the circulation of a coolant. In order to reach the regions of the ingot mold in which the temperature needs to be measured, it is necessary to pass through this cooling device and thus through the circulating water. This causes further problems of sealing and wiring.

An additional problem that is encountered with prior art ingot molds provided with measurement devices is also linked to the very limited accessibility of the ingot mold, and in particular the walls thereof, while it is being used. It would be particularly advantageous to be able to have an ingot mold in which the temperature measurement device can, in the event of failure, be replaced without it being necessary to disassemble the entire plant.

WO-A1-2017/032488 discloses an ingot mold for continuous metal casting. The ingot mold is formed by an assembly of four copper plates 10, at least one of these plates having a plurality of ducts 12 that each receive an optical fiber 20. They make it possible to measure the temperature of the metal being cast in the ingot mold, the document citing in particular the “Fiber Bragg Grating” method. In the embodiment illustrated in FIG. 1c , the optical fibers 20 extend perpendicular to the casting direction of the metal. However, the ducts disclosed in that document have a diameter of between 0.3 and 1.2 mm and never have a diameter greater than 1.2 mm.

It is an aim of the invention to improve the detection of breakout by remedying the drawbacks set out above.

To this end, the invention provides an ingot mold for continuous metal casting, of the type formed by an assembly of metal plates bearing on cooling devices configured to allow the cooling of the metal plates by the circulation of a coolant, comprising an optical fiber, having a plurality of Bragg filters, extending in a wall of at least one of said plates, the optical fiber extending in a direction that is not parallel to the casting axis of the ingot mold, wherein the optical fiber has a diameter greater than 1.6 mm. The diameter of the optical fiber takes into account the optional presence of a coating, of a cladding, of a tube or of a combination thereof (for example, the core may be provided with a thin cladding, itself inserted into a tube, itself provided with a coating). In other words, the diameter of the optical fiber is the diameter of the assembly made up of the core, the cladding and, as the case may be, the coating or the tube or a combination thereof.

Thus, the prior art thermocouples are replaced with an optical fiber comprising Bragg filters. The latter make it possible, by means of the emission of a light beam through the fiber and the detection of the reflected and/or transmitted beam, to measure the temperature in the wall at each of the filters. It will be understood that the optical fiber takes up less space than the thermocouples and is much easier to fit. In addition, the temperature measurement by virtue of the Bragg filters is more precise than that obtained with the thermocouples, and so the number of false alarms is reduced.

Moreover, by providing the optical fiber with a sufficiently large diameter, it is much easier to manufacture the ingot mold, more particularly to prepare a duct in the wall into which the optical fiber is inserted. This is because it is industrially difficult to precisely bore a very long duct with a small diameter. The diameter of the duct should be approximately the same as that of the optical fiber in order that there is no uncertainty as regards the actual position of the fiber in the duct, which would make the measurement of the temperature inaccurate. By increasing the diameter of the optical fiber, it is possible to increase the diameter of the duct and thus make it easier to prepare.

Advantageously, the optical fiber is provided with a coating or with a tube.

It is thus possible to easily increase the diameter of the optical fiber if necessary.

Advantageously, the optical fiber has a diameter greater than or equal to 2 mm, preferably greater than or equal to 2.5 mm, preferentially equal to 3 mm.

Advantageously, the direction exhibits an angle of between 75° and 105° with the casting axis.

Advantageously, the ingot mold has a square or rectangular cross section.

The plant thus makes it possible to produce a metal slab with a square or rectangular section, this generally being convenient for the subsequent use that is made of the slab.

Preferably, the ingot mold comprises optical fibers in at least two mutually opposite plates, preferably in four plates.

Advantageously, the ingot mold is made of copper or of a copper alloy.

The ingot mold is thus made of a highly thermally conductive material. This facilitates exchanges of heat between the cooling devices and the metal passing through the ingot mold.

Advantageously, the optical fiber is installed bare in the ingot mold.

According to one embodiment variant, the optical fiber is provided with a coating.

According to another embodiment variant, the optical fiber is inserted into a tube extending in a wall of at least one of said plates.

Thus, it is possible to vary the diameter of the optical fiber by virtue of the presence or absence of a coating or of a tube. This allows more freedom in the dimensioning of the duct in the wall of the plate of the ingot mold and thus makes it easier to create.

Advantageously, the ingot mold comprises optical fibers in at least two mutually opposite plates, preferably in four plates.

This improves the measurement of the temperature in the walls of the ingot mold, making it possible to make the detection of breakout more reliable.

According to one embodiment variant, the ingot mold comprises a single optical fiber.

The ingot mold is thus easy to produce and has a modest manufacturing cost.

Advantageously, the optical fiber has at least ten Bragg filters per meter, preferably at least twenty Bragg filters per meter, preferentially at least thirty Bragg filters per meter, and even more preferentially at least forty Bragg filters per meter.

It is thus possible to measure the temperature in the walls of the ingot mold at a large number of points, thereby helping to make the detection of breakout more reliable.

Advantageously, the ingot mold comprises at least two optical fibers that extend parallel to one another and are preferably spaced apart by between 10 and 25 centimeters, more preferentially spaced apart by between 15 and 22 centimeters.

It is thus possible to measure the temperature of the walls of the ingot mold at two different heights of the ingot mold. This is particularly effective since it makes it possible to better monitor the propagation of the sticking phenomenon along the ingot mold, and therefore to better determine if a breakout is likely to arise.

The invention also provides an ingot mold for continuous metal casting, of the type formed by an assembly of metal plates bearing on cooling devices configured to allow the cooling of the metal plates by the circulation of a coolant, which has at least one duct extending in a direction that is not parallel to a casting axis of the ingot mold, in a wall of at least one of said plates, wherein the duct has a diameter greater than or equal to 1.6 mm.

Advantageously, the duct has a diameter greater than or equal to 2 mm, preferably greater than or equal to 2.5 mm, preferentially equal to 3 mm.

According to a first embodiment, the duct is a through-duct.

According to a second embodiment, the duct leads out at only one lateral end of the plate.

According to a third embodiment, each of the ducts extending along at least half the length of the plate and leading out at two opposite lateral ends of the plate.

According to a fourth embodiment, the ingot mold has two, non-communicating, coaxial ducts that lead out at two opposite lateral ends of the plate.

Preferably, the duct(s) is/are created by drilling into the wall of the plate.

According to one embodiment variant, the duct(s) is/are created by recessing, for example by milling, one or more grooves into the wall of the plate and then by sealing an upper part of the grooves.

These different embodiments correspond to as many means of installing the optical fiber in the ingot mold, thereby showing the versatility of the invention.

The invention also provides a system for detecting breakout in a continuous metal-casting system, comprising:

-   -   an ingot mold as defined above,     -   a transceiver designed to send light into the optical fiber and         receive reflected light and/or light transmitted by the optical         fiber,     -   a processor designed to convert data relating to the reflected         and/or transmitted light received by the transceiver into         information about the detection of a breakout, and     -   a terminal comprising a user interface, said terminal being         connected to the processor.

The invention also provides a method for detecting a breakout in a continuous metal-casting plant, characterized in that the temperature of a wall of an ingot mold as defined above is measured.

An embodiment of the invention will now be presented, said embodiment being given by way of nonlimiting example and with reference to the appended figures, in which:

FIG. 1 is an overall view of a continuous metal-casting plant comprising an ingot mold according to the invention,

FIGS. 2a and 2b are diagrams illustrating the functioning of the plant in FIG. 1,

FIG. 3 is a view in section of the ingot mold of the plant in FIG. 1,

FIG. 4 is a perspective view of a plate of the ingot mold in FIG. 3,

FIG. 5 is a view in longitudinal section of an optical fiber contained in the wall in FIG. 4,

FIG. 6 is a diagram explaining the functioning of the optical fiber in FIG. 5, and

FIGS. 7a, 7b, 7c and 7d are views in section of the ingot mold in FIG. 3, illustrating the creation of a breakout.

FIG. 1 shows a continuous metal-casting plant 2. It has a conventional configuration, and so most of the constituent elements thereof will be presented only briefly.

The plant 2 comprises ladles 4 containing liquid metal that is intended to be cooled. In this case, there are two ladles 4, which are carried by a motor-driven arm 6. This motor-driven arm 6 is in particular able to move the ladles 4, which are moved in a full state into the casting zone by a transportation system (for example a bridge crane, not shown) from a filling zone in which the molten metal can be poured into said ladles, for example a furnace or a converter (not shown), before they are moved into the position illustrated in FIG. 1. After the ladle 4 has been emptied, the motor-driven arm 6 also makes it possible to position the empty ladle in a position in which the transportation system can retrieve it and move it into a preparation zone in which it will be reconditioned before returning to the filling zone.

The plant 2 comprises a tundish or distribution basin 8 situated beneath the ladles 4. The latter have an openable bottom to allow the liquid metal to be poured into the tundish 8.

The tundish 8 comprises a flow orifice that can be closed off by a stopper rod 10, which makes it possible to control the flow of liquid metal. The flow orifice of the tundish is continued by a submerged entry nozzle 11 (SEN) for protecting the liquid metal poured into the ingot mold 12.

As can be seen more clearly in FIG. 2a and on a larger scale in FIG. 2b , the submerged entry nozzle 11 leads into an upper opening of an ingot mold 12. This is a bottomless ingot mold having a casting axis that is vertical. The ingot mold 12 will be described in more detail below.

The plant 2 comprises cooling devices 14 positioned on an external surface of the ingot mold 12. These are liquid-type cooling devices. To this end, they comprise conduits through which a refrigerant, for example water, flows. The refrigerant absorbs the heat of the liquid metal in the ingot mold 12 in order to cool and solidify it. In this case, the metal solidifies in the form of a slab having a solidified external surface 18 enclosing a liquid core 20.

The plant 2 comprises a roller guide 16 located downstream of the ingot mold 12. The guide 16 makes it possible to guide the slab, an external surface 18 of which is solidified, out of the ingot mold 12. As can be seen in FIG. 2a , the slab solidifies gradually as it moves in the guide 16. In other words, the further it moves away from the ingot mold 12, the more the volume of the solidified external surface 18 of the slab increases and the more the volume of the liquid core 20 of the slab decreases.

The ingot mold 12 is shown in more detail in FIG. 3. In this case, it has four plates 22 (the fourth not being visible on account of the position of the section plane). The plates 22 are made of copper or copper alloy, these being materials that are highly thermally conductive and thus facilitate the exchanges of heat between the cooling devices 14 and the ingot mold 12. The plates 22 are arranged such that the ingot mold 12 has a rectangular or square cross section. However, the plates could also be arranged such that the ingot mold has any other cross-sectional shape.

One of the plates 22 of the ingot mold 12 is shown on a larger scale in FIG. 4, in which the casting axis corresponds to the vertical direction. In its wall, the plate 22 has at least one duct 24 extending in a direction that is not parallel to the casting axis of the ingot mold 12. More specifically, the duct 24 exhibits an angle of between 75° and 105° with the casting axis. In this case, the duct 24 is perpendicular to the casting axis. The duct 24 has a diameter greater than or equal to 1.6 mm. Preferably, the duct 24 has a diameter greater than or equal to 2 mm, preferentially greater than or equal to 2.5 mm. In this case, it has a diameter of 3 mm. There are four ducts 24 in this case. A protective cover 26 is installed over the region of the plate 22 at which the ducts 24 lead out in order to protect them.

In this first embodiment of the invention, the ducts 24 are through-ducts. According to a second embodiment of the invention, the ducts lead out at only one lateral end of the plate. According to a third embodiment of the invention, the plate has two ducts that are parallel but not coaxial, each of the ducts extending along at least half the length of the plate and leading out at two opposite lateral ends of the plate. According to a fourth embodiment of the invention, the plate has two, non-communicating, coaxial ducts that lead out at two opposite lateral ends of the plate.

The ducts 24 are created by drilling into the wall of the plate 22. In a variant, however, it is possible to provide for the ducts to be created by recessing one or more grooves into the wall of the plate and then by sealing an upper part of the grooves.

An optical fiber 28 is accommodated in each of the ducts 24. The optical fiber 28 has a diameter that is approximately the same as the diameter of the duct 24. The optical fiber 28 has a diameter greater than or equal to 1.6 mm. Preferably, the optical fiber 28 has a diameter greater than or equal to 2 mm, preferentially greater than or equal to 2.5 mm. In this case, it has a diameter of 3 mm, like the duct 24. However, a tolerance may be provided between the diameters of the duct 24 and of the optical fiber 28, for example less than 0.1 mm or even less than 0.05 mm. With reference to FIGS. 5 and 6, each optical fiber 28 comprises an optical cladding 30 and a core 32 surrounded by the optical cladding 30. The optical fiber 28 comprises a plurality of Bragg filters 34 in its core 32. The optical fiber 28 has at least ten Bragg filters per meter, preferably at least twenty Bragg filters per meter, preferentially at least thirty Bragg filters per meter, and even more preferentially at least forty Bragg filters per meter. In an embodiment variant, provision could be made for the ingot mold to contain only one optical fiber.

The optical fiber 28 may either be accommodated bare in the duct 24 or be provided with a protective coating or be inserted into a tube before being installed. As mentioned above, the diameter of the optical fiber 28 takes into account the optional presence of a coating or of a tube. In other words, the diameter of the optical fiber 28 is the diameter of the assembly made up of the core 32, the optical cladding 30 and, as the case may be, the coating or the tube. This coating or tube may specifically have the function of increasing the radius of the optical fiber 28 so as to fill the diameter of the duct 24 completely or almost completely. This is because it is relatively difficult to drill a small-diameter duct along a great length. Hence, increasing the diameter of the optical fiber 28 makes it possible to increase the possible diameter of the duct 24 and thus to make it easier to produce.

The functioning of the optical fiber 28 is illustrated in FIG. 6. The Bragg filters 34 are filters that make it possible to reflect the light over a wavelength range centered on a predetermined value, referred to as reflected wavelength, which is able to be set by the filter manufacturer. This predetermined value furthermore depends in particular on the temperature at which the filter is, such that, for each filter, it is possible to state:

λ_(reflected) =f(λ₀ ,T)

-   -   where λ_(reflected) is the wavelength effectively reflected by         the filter, f is a known function, T is the temperature of the         filter and λ₀ is the wavelength reflected by the filter at a         predetermined temperature, for example at ambient temperature.

These two properties make it possible to use the optical fiber 28 as a temperature sensor. First of all, Bragg filters 34 having different and selected reflected wavelength values λ₀, separated from one another for example by 5 nanometers, are installed in the optical fiber 28. Next, a light beam having a polychromatic spectrum 35 a, for example white light, is sent into the optical fiber 28 and then the wavelength peaks represented in the spectrum of the reflected beam 35 b are determined. At each peak, the measured value λ_(reflected) and the theoretical value of the wavelength reflected at ambient temperature λ₀ are compared, and the temperature T of the filter in question is calculated by virtue of the function f. Alternatively, it is also possible to carry out these steps on the basis of troughs in the spectrum of the transmitted beam 35 c if the configuration of the duct 24 in which the optical fiber 28 is accommodated allows this.

Thus, the installation of optical fibers 28 in the walls of the ingot mold 12 makes it possible to measure the temperature of these walls at predetermined positions as it changes over time. In order to obtain a sufficient number of measurement points, it is preferred to place at least one optical fiber 28 in each of the four plates 22 of the ingot mold 12. However, a more economical solution would be to place optical fibers 28 only in two opposite plates 22.

Furthermore, it is also preferred to place two optical fibers 28 per plate 22 so as to be able to measure the temperature of the ingot mold 12 at two different heights. For example, it is possible to place the two optical fibers 28 in each plate such that they are parallel and spaced apart by 15 to 25 centimeters.

A breakout is detected in the following way.

FIGS. 7a to 7d show the propagation of a region 36 in which the metal contained in the ingot mold 12 sticks to one of the plates 22 thereof. The graphs located in the bottom right-hand region of each of these figures show the change in the temperature measured by a Bragg filter 34 of an upper optical fiber 28 a (top curve) and by a Bragg filter 34 of a lower optical fiber (28 b) over time.

As can be seen in the graphs in FIGS. 7a and 7b , the upper optical fiber 28 a detects an abnormal increase in temperature, which corresponds to the sticking of the metal to the ingot mold 12 in the region 36. This is a first sign of an imminent breakout.

Next, as can be seen in the graphs in FIGS. 7c and 7d , the lower optical fiber 28 b detects the abnormal increase in temperature previously detected by the upper optical fiber 28 a. This is a second sign of an imminent breakout, providing confirmation that the breakout does not appear to be avoidable.

In order that the information picked up by the optical fibers 28 a and 28 b is communicated to the users of the plant 2, the latter comprises:

-   -   a transceiver designed to send light into the optical fibers and         receive reflected light and/or light transmitted by the optical         fibers,     -   a processor designed to convert data relating to the reflected         and/or transmitted light received by the transceiver into         information about the detection of a breakout, and     -   a terminal comprising a user interface, said terminal being         connected to the processor.

By virtue of these elements (which have not been shown in the figures for reasons of clarity), it is possible to convert the temperature measurement carried out by the optical fibers 28 into information, which is understandable by the users of the plant 2, about the detection or non-detection of a breakout. In other words, the ingot mold 12 equipped with optical fibers 28, the transceiver, the processor and the terminal form a breakout detection system. In the event of a positive detection of a breakout, the users can act to reduce the damage caused by the breakout or even to prevent it.

The invention is not limited to the embodiments presented and other embodiments will be clearly apparent to a person skilled in the art.

NOMENCLATURE

-   -   2: Plant (for continuous metal casting)     -   4: Ladle     -   6: Motor-driven arm     -   8: Tundish     -   10: Stopper rod     -   11: Nozzle     -   12: Ingot mold     -   14: Cooling devices     -   16: Guide     -   18: Solidified external surface     -   20: Liquid core     -   22: Plate     -   24: Duct     -   26: Protective cover     -   28: Optical fiber     -   30: Optical cladding     -   32: Core     -   34: Bragg filter     -   35 a: Polychromatic spectrum     -   35 b: Spectrum of the reflected beam     -   35 c: Spectrum of the transmitted beam     -   36: Region 

1. An ingot mold (12) for continuous metal casting, of the type formed by an assembly of metal plates (22) bearing on cooling devices (14) configured to allow the cooling of the metal plates (22) by the circulation of a coolant, comprising an optical fiber (28), having a plurality of Bragg filters (34), extending in a wall of at least one of said plates (22), the optical fiber (28) extending in a direction that is not parallel to the casting axis of the ingot mold (12), wherein the optical fiber (28) has a diameter greater than 1.6 mm.
 2. The ingot mold (12) as claimed in the preceding claim, wherein the optical fiber (28) is provided with a coating or with a tube.
 3. The ingot mold (12) as claimed in the preceding claim, wherein the direction exhibits an angle of between 75° and 105° with the casting axis.
 4. The ingot mold (12) as claimed in the preceding claim, which has a square or rectangular cross section and comprises optical fibers (28) in at least two mutually opposite plates (22), preferably in four plates (22).
 5. The ingot mold (12) as claimed in the preceding claim, wherein the optical fiber (28) has at least ten Bragg filters per meter, preferably at least twenty Bragg filters per meter, preferentially at least thirty Bragg filters per meter, and even more preferentially at least forty Bragg filters per meter.
 6. The ingot mold (12) as claimed in the preceding claim, which comprises at least two optical fibers (28) that extend parallel to one another and are preferably spaced apart by between 10 and 25 centimeters, more preferentially spaced apart by between 15 and 22 centimeters.
 7. An ingot mold (12) for continuous metal casting, of the type formed by an assembly of metal plates (22) bearing on cooling devices (14) configured to allow the cooling of the metal plates (22) by the circulation of a coolant, the ingot mold having at least one duct (24) extending in a direction that is not parallel to a casting axis of the ingot mold (12), in a wall of at least one of said plates (22), wherein the duct has a diameter greater than or equal to 1.6 mm.
 8. The ingot mold as claimed in claim 7, wherein the duct has a diameter greater than or equal to 2 mm, preferably greater than or equal to 2.5 mm, preferentially equal to 3 mm.
 9. The ingot mold (12) as claimed in claim 7, wherein the duct leads out at only one lateral end of the plate.
 10. The ingot mold (12) as claimed in claim 7, which has two ducts that are parallel but not coaxial, each of the ducts extending along at least half the length of the plate and leading out at two opposite lateral ends of the plate.
 11. The ingot mold (12) as claimed in claim 7, which has two, non-communicating, coaxial ducts that lead out at two opposite lateral ends of the plate.
 12. The ingot mold (12) as claimed in claim 7, wherein the duct(s) is/are created by drilling into the wall of the plate.
 13. The ingot mold (12) as claimed in claim 7, wherein the duct(s) is/are created by recessing one or more grooves into the wall of the plate and then by sealing an upper part of the grooves.
 14. A system for detecting breakout in a continuous metal-casting system, comprising: an ingot mold (12) as claimed in claim 1, a transceiver designed to send light into the optical fiber (28) and receive reflected light and/or light transmitted by the optical fiber (28), a processor designed to convert data relating to the reflected and/or transmitted light received by the transceiver into information about the detection of a breakout, and a terminal comprising a user interface, said terminal being connected to the processor.
 15. A method for detecting a breakout in a continuous metal-casting plant, wherein the temperature of a wall of an ingot mold (12) as claimed in claim 1 is measured. 